I'm not sure why that is disabled but I think that looks like a correct behavior. The reason could be that once you click on run the mesh should be generated before it should be solved
This is the intended behavior. For calculating the results of a CFD analysis mesh should be generated in order to capture the geometry and consider the applied boundary conditions. In the attached image it is seen that the mesh details are not available [Results(Not Loaded)] for this project. In order to Run the study for the first time meshing is mandatory and hence the check box is grayed out for the initial run.
But when you un-check the Run and select mesh only, after meshing is done you will see that a *.cpt mesh file is shown next to the Results node.
When you have mesh file linked to the study and if you try to Run the study you will have access to both Mesh only and Run only.
There could be another scenario where in when you modify the geometry after creating the project in flow simulation, the tool is smart enough to detect the geometrical changes and it will ask the user to update the computational domain. Finally it will force the user to remesh the body and run the analysis.
Hope this answers your question.